The present invention relates to actively controlled suspension systems and, in particular, to an active suspension system having an active mode and a passive mode.
Active suspension systems are known to those skilled in the art. In general, active suspension systems control a vehicle's "ride" and "handling". Active suspension systems control and maintain a vehicle's attitude, for example, elevation, roll and pitch. Active suspension systems sense the attitude of the car and, in response, provide power to or remove power from actuators to maintain and control the "ride" and "handling" of the vehicle. For example, Williams et al., U.S. Pat. No. 4,625,993, teaches of a conventional active suspension system intended to maintain a vehicle substantially stable in all planes of movement irrespective of any external forces acting upon the vehicle.
Active suspension systems, for example in automobiles and the like, include a pressurized fluid source and suspension units. The pressurized fluid source, for example a hydraulic pump, provides fluid power in the form of a pressurized working fluid to suspension units at each vehicle wheel. The suspension units dissipate and/or counteract undesirable external forces.
The fluid power provided by the fluid source is critical to the performance of the active suspension system. The response of the active suspension system depends upon the suspension units having sufficient fluid power to dissipate and/or counteract undesirable external forces. When there is a system failure, for example an interruption in fluid communication between the pump and the suspension units, the system is unable to function in the active mode. Under these conditions, if the suspension system is to provide any suspension at all, the system will be required to function in a passive mode as a passive-like suspension system.
In addition to a loss of the active mode, an interruption in fluid communication between the pump and the suspension units can cause serious damage to various elements of the suspension system. The pump communicates with each of the suspension units through conduits, for example, a main conduit coupled to the pump and secondary conduits which "feed off" the main conduit and supply each suspension unit with the pressurized fluid. An interruption in fluid communication can result from a break in the main conduit which would lead to a complete loss of fluid in each of the suspension units. This could cause serious damage to various elements of the suspension units as well as the pump.
To prevent such damage and still provide suspension for the vehicle, suspension units have been proposed which include protection valves and by-pass valves. In suspension systems having double acting actuators, for example a cylinder having two chambers, in the event of a system failure, the protection valve fluidly isolates the suspension unit from the remaining suspension system. In doing so, the protection valve traps the pressurized fluid within the suspension unit. The by-pass valve then provides fluid communication between the two chambers. Further, the by-pass valve often provides some resistance to the flow of fluid between the two chambers. These valves have been employed in various configurations and can include supporting valves as well.
For example, active suspension systems like the system disclosed in West German patent DE 3638-574A employ a 3-port, two state, switching valve and a 4-port, two state, power valve. These valves are configured such that if there is a loss of fluid pressure or if there is an electrical power failure, the pump is disconnected from the suspension unit and a fluid restrictor is incorporated between the two chambers to provide resistance to fluid flow between the chambers. In this configuration, the suspension functions in a passive mode.
Active suspension systems like the system disclosed in DE 3638-574A employ a 3-port switching valve to both isolate the suspension unit and provide resistance to the flow of fluid when in the passive mode. The switching valve is pressure actuated and is responsive to the pressure of the fluid near the pump. Here the switching valve is responsive to the pressure of the fluid in the main pump fluid conduit instead of the pressure of the fluid in or approximate to the suspension unit.
Further, systems of this type incorporate the restricting portion within the fluid communication path in the passive mode only. These systems employ a fluid restrictor that, when interposed between the two chambers of the cylinder, provides a fixed resistance regardless of, for example, the current conditions of the road texture. The valve spool of the restrictor is set in a constant predetermined position--often when installed in the vehicle.
There exists a need for a valve configuration for converting an active suspension system into a passive suspension system having a protection valve to fluidly isolate the suspension unit and a separate by-pass valve to restrict fluid flow between the chambers of the cylinder. There exists a need for a by-pass valve that is fluidly coupled to the protection valve and the cylinder in both the active and passive modes while allowing restricted fluid flow therethrough when in the passive mode.
There exists a need for a valve configuration for converting an active suspension system into a passive suspension system having a protection valve which is pressure actuated and responsive to the pressure of the fluid in the suspension unit. Further, there exists a need for the by-pass valve having an adjustable fluid resistance which is set according to current suspension conditions, for example, according to the current "texture" of the road.
There exists a need for a protection valve that automatically isolates the suspension unit under conditions other than loss of supply pressure, for example a loss of electrical power and/or a malfunction of a control unit.